1. Technical Field
The present invention relates to a light emitting device having a straight-line shape using an electroluminescent element.
2. Description of the Related Art
A conventional semiconductor light emitting element operates at a low voltage and has high brightness. However, since the element is a light source having a spot shape, it is difficult to use the element as a light source having a straight-line shape or a light source having a plane shape. Furthermore, an expensive substrate is necessary for the fabrication of a light emitting element, which is one factor that increases the cost. In addition, in the case of a thin film type light emitting element, a Schottky barrier is generated at the interface at which a phosphor layer and an electrode are joined, and there is a problem where the injection of carriers is prevented.
FIG. 14 is a schematic configuration diagram showing a configuration of a conventional light emitting element 50. A phosphor layer 53 has a configuration of a recombination type phosphor layer and the phosphor layer 53 having a two-layer structure of an n type semiconductor layer 53a and a p type semiconductor layer 53b is provided. A transparent electrode 52 which is functioned as an electron injecting electrode and a rear surface electrode 54 which is functioned as a hole injecting electrode are electrically connected via a direct current source 55. When power is supplied from the direct current source 55, a potential difference is generated between the transparent electrode 52 and the rear surface electrode 54 and a voltage is applied across the phosphor layers 53a and 53b. Thus, the phosphor layers 53a and 53b placed between the transparent electrode 52 and the rear surface electrode 54 emit light, and the light transmits through the transparent electrode 52 to be emitted to the outside of the light emitting element 50.
Here, depending on the combination of a semiconductor and an electrode, a Schottky barrier is generated at the interface through which the two are joined, the efficiency of injection of electrons and holes into the phosphor layer 53a and 53b, respectively, is decreased. Thus, the system is prevented from becoming more efficient. The problems concerning the Schottky barrier at this joint interface are described in reference to energy band diagrams in FIGS. 15A, 15B, 16A and 16B.
FIGS. 15A and 15B are energy band diagrams before and after contact in the case where an n type semiconductor layer 53a and a transparent electrode 52 are brought into contact with each other. Before contact, as shown in FIG. 15A, the semiconductor and the electrode exhibit different Fermi levels relative to the vacuum level. When the semiconductor and the electrode are brought into contact with each other, as shown in FIG. 15B, the band of the n type semiconductor layer 53a is curved on the contact surface so that the respective Fermi levels coincide with each other, and a large Schottky barrier is generated between the n type semiconductor layer 53a and the transparent electrode 52 after contact. Therefore, the efficiency of injection of electrons from the transparent electrode 52 to the n type semiconductor layer 53a is decreased. In addition, a metal oxide, such as ITO, for example, is used as the transparent electrode 52. Since the work function of such a material is generally relatively large, for example, 4 eV to 5 eV, a large Schottky barrier is generated between the n type semiconductor layer 53a and the transparent electrode 52.
In addition, FIGS. 16A and 16B are energy band diagrams before and after contact in the case where a p type semiconductor layer 53b and a rear surface electrode 54 are brought into contact with each other. In the case of the p type semiconductor layer 53b, when the semiconductor and the electrode are brought into contact with each other in the same manner as in the case of the n type semiconductor layer 53a, the band of the p type semiconductor layer 53b is curved on the contact surface so that the respective Fermi levels coincide with each other. Therefore, as shown in FIG. 16B, a large Schottky barrier is generated between the p type semiconductor layer 53b and the rear surface electrode 54 and the efficiency of injection of holes from the rear surface electrode 54 to the p type semiconductor layer 53b is decreased.
In order to solve the above-described problems, the following methods are generally used.
(1) A material having a large work function is used as a hole injecting electrode.
In addition, a material having a small work function is used as an electron injecting electrode.
(2) A layer which is doped with a high concentration is formed in an interface between an electrode and a semiconductor (see, for example, Japanese Patent Laid-open Publication No. 2005-294415, and J. Crystal Growth 214/215, p. 1064 (2000)).
(3) A Schottky barrier is made smaller through a reaction in which an electrode material and a semiconductor form an alloy (see, for example, Solid-State Electronics, Vol. 42, No. 1, pp 139-144, 1998).